Fibrous carboxyalkyl cellulose derivatives having a sufficiently low degree of carboxyalkyl substitution (D.S.) so that they are essentially water insoluble have been known for many years. The carboxymethyl and carboxyethyl cellulose derivatives of this type have been the principal compositions investigated. These materials have been considered as paper making fibers although their usage has not been extensively investigated.
Walecka, in an article in Tappi, 39 (7): 458-463 (1956), describes a method for making carboxymethylated cellulose in the D.S. range of 0.006-0.62. Freeness of a water suspension of his carboxymethylated fibers at a given beating time was seen to decrease with increasing D.S. Bursting, tensile strength, and zero span tensile strength of handsheets made at a given beating time increased with increasing D.S. while opacity was observed to decrease. In summary, the author noted that the modified fibers were easier to fibrillate both internally and externally during beating and tended to form sheets with superior strength properties.
Baker and Sepall, Pulp and Paper Magazine of Canada, pp T-449 to T-455 (September, 1965) describe low D.S. carboxyethylation of wood pulps and other cellulosic materials. They reacted the cellulosic material with varying amounts of acrylamide and sodium hydroxide at relatively high consistency in an aqueous environment. The resulting pulps were described as "carbamylethyl-carboxyethyl derivatives with the pre-dominating substituent determined by caustic concentration". Higher caustic concentrations favored formation of the carboxyethyl derivative. The authors concluded that it was only the carboxyethyl substituent that affected papermaking properties and their reported D.S., which ranged between about 0.02-0.21, referred only to this group. Increasing carboxyethylation had a positive effect on bursting strength but a negative effect on unbeaten freeness and tear strength. Bulk was generally increased as D.S. increased. The higher D.S. pulps, those in the range of 0.18 were particularly noted, tended to be quite slimy feeling in aqueous slurries. The authors found it surprising that the modified pulps produced "strong paper of unusually low density" and particularly mentioned the apparent ability to repulp and reform previously dried sheets without significant loss of strength.
Burova et al., Leningrad Tekhnol. Inst. Tsellyul.-Bumazh. Prom.30: 61-70 (1970), investigated conditions for preparation of low D.S. carboxyethyl cellulose. They confirmed the observations of Baker and Sepall that increasing concentrations of caustic directed the reaction to carboxyethyl substitution rather than carbamoylethyl. They further agreed that this latter substituent did not affect papermaking properties of the unreacted fiber. Handsheets formed from fibers with varying degree of substitution led them to the observation that any significant change in the mechanical properties of paper were not seen until carboxyethyl substitution was significant, in their case about 0.056, the highest D.S. they achieved. Fold, burst, and water absorption capability (hydrophilicity) were improved over untreated fiber, bulk was also increased, but tear strength dropped. The authors also noted the easier beating properties and retention of strength after repulping that had been seen by Baker and Sepall.
German Offenlegungssehrift 1,932,753 describes absorptive paper products made using a blend containing a predominant mount (at least 50%) of relatively low D.S. insoluble carboxymethyl cellulose fibers and a wet strength additive. The addition of more than 50% untreated fibers was said to degrade the properties very significantly. While carboxyethyl cellulose was mentioned as one of a large family of substituted cellulose products that might be suitable there were no examples of its use or of any of the many other suggested materials other than carboxymethylated fibers. A fairly high degree of carboxymethyl substitution appeared to be necessary to gain the improved properties claimed. A D.S. greater than 0.05 was required, preferably greater than 0.10. In some examples a D.S. as high as 0.20 appeared to work best. Various conventional wet strength resins were suggested for use with the fiber. These included urea formaldehyde and melamine urea formaldehyde types, with polyethyleneimine and polyamide epichlorohydrin resins being preferred. Paper towels and wet wipes were among the products suggested.
Neogi and Jensen, Tappi 63 (8): 86-88 (1980), note that a higher degree of ionic bonding of positively charged (cationic) additives can be effected in papers by introduction of "large mounts of negatively charged fibers such as carboxymethyl cellulose". In one example, 5% CMC, incorporated with an unbleached kraft pulp along with a polyethyleneimine wet strength additive, increased wet tensile strength by a factor of 2 over a similar material lacking the carboxymethylated material. However, the CMC used had a D.S. of 0.7 and would appear to be a water soluble material and not one that would remain in fibrous form. Additionally, the authors note that the polyethyleneimine in aqueous solution was sprayed onto the surface of previously formed sheets rather than by the more standard practice of being incorporated into the furnish before sheeting.
Wagberg and Bjorkland, Nordic Pulp and Paper Research Journal, No. 1, pp 53-58 (1993), also studied the use of low D.S. carboxymethylated fibers with wet strength enhancers. They prepared substituted fibers in a D.S. range of 0.016-0.126 using the method of Waleeka cited above. Wet and dry tensile strengths were the principal sheet properties investigated. These were studied at various fiber D.S. levels and various levels of added polyamide epichlorohydrin (PAE) wet strength additive. It was noted that higher D.S. fiber absorbed greater amounts of PAE resin but that both wet and dry tensile index leveled off at a D.S. of about 0.07. This was believed due to a higher degree of swelling of the higher D.S. fibers that permitted the resin to enter the interior of the fibers where it would not be effective. At a constant PAE addition of 4 kg/t, dry tensile index increased roughly linearly with increasing D.S. This was also the case for a control set in which no PAE had been added. Wet tensile index followed a similar pattern of increasing strength as D.S. increased for the sheets with the added PAE. However, little or no increase was observed in the comparison set made without the wet strength additive. The wet tensile index values of sheets with the PAE resin were significantly higher than those lacking the additive. The authors noted that dry tensile index is affected by both D.S. of the fibers and the addition of PAE but that the effect of change in D.S. is greater than the effect of change in the amount of wet strength resin used. In conclusion they stated that an increase in the carboxyl group content led to an increase in the ability to absorb wet strength resins and that a chemical reaction probably took place between them. They further concluded that the added carboxyl groups enhanced the efficiency of the wet strength resin.
Ward, in his book Chemical modification of Papermaking Fibers, Marcel Decker, Inc., New York (1973), quotes extensively from the Waleeka and Baker and Sepall papers just noted. He further cites Kapustova and Letenay, Papir Celuloza 25 (6):171-174 (1970) [Abstract Bulletin of the Institute of Paper Chemistry, 41: 6461 (1971)] on the use of CMC in pulp sheets. In one case 1-5% CMC (apparently a water soluble type) was precipitated on prehydrolyzed kraft pulp fibers by alum prior to beating. Refining properties, breaking length, bursting strength, and folding endurance were said to be improved. In another example, high wet strength was achieved in sheets made of low substituted (water insoluble) CMC. Ward further cites Letenay, Sb. Vyskum. Prac. Odboru Papiera Celulozy 13: 81-90 (1968) [Abstract Bulletin of the Institute of Paper Chemistry, 40: 5749 (1970) who added up to 10% of hydrophilic cellulose derivatives as additives to a spruce kraft pulp. These additives included carboxymethyl and carboxyethyl cellulose. Refining properties were initially improved with use of the additives. Handsheets made from pulp to which 10% CMC had been added showed an increase in fold endurance but reduced breaking length and burst compared to sheets lacking the modified fiber.
While it would be anticipated that insoluble carboxymethylated and carboxyethylated fibers of similar carboxyl equivalent would behave in similar fashion, the present inventors have unexpectedly found that this not the case. Surprisingly, no one until the present has discovered the benefits and the very significant product advantages obtained by the use of carboxyethylated fibers as at least a portion of a papermaking furnish when used in conjunction with various cationic additive materials.